The history of Norway has been influenced to an extraordinary degree by the terrain and the climate of the region. About 10,000 BC, following the retreat of the great inland ice sheets, the earliest inhabitants migrated north into the territory which is now Norway. Between 5,000 BC and 4,000 BC the earliest agricultural settlements appeared around the Oslofjord. Gradually, between 1500 BC and 500 BC, these agricultural settlements spread into the southern areas of Norway – whilst the inhabitants of the northern regions continued to hunt and fish. The Neolithic period started in 4000 BC. Norway entered the Kalmar Union with Denmark and Sweden in 1397. After Sweden left the union in 1523, Norway became the junior partner in Denmark–Norway. In 1814, after being on the losing side of the Napoleonic Wars with Denmark, Norway was ceded to the king of Sweden by the Treaty of Kiel. Norway declared its independence and adopted a constitution. However, no foreign powers recognized the Norwegian independence but supported the Swedish demand for Norway to comply with the treaty of Kiel. After a short war with Sweden, the countries concluded the Convention of Moss, in which Norway accepted a personal union with Sweden, keeping its Constitution, Storting and separate institutions, except for the foreign service. The union was formally established after the extraordinary Storting adopted the necessary amendments to the Constitution and elected Charles XIII of Sweden as king of Norway on 4 November 1814. Industrialization started in the 1840s and from the 1860s large-scale emigration to North America took place. In 1884 the king appointed Johan Sverdrup as prime minister, thus establishing parliamentarism. The union with Sweden was dissolved in 1905. From the 1880s to the 1920s, Norwegians such as Roald Amundsen and Fridtjof Nansen carried out a series of important polar expeditions.